1. Field of the Invention
The present invention relates to a magnetic recording medium having servo signal unit portions formed in a concavo-convex shape, a magnetic recording/reproducing apparatus with the magnetic recording medium, and a stamper for manufacturing the magnetic recording medium.
2. Description of the Related Art
Conventional magnetic recording media such as hard disks have been used with the recording layer divided into a plurality of data areas and a plurality of servo areas. In the servo area, control servo signals such as for positioning a magnetic head are magnetically recorded in a predetermined servo pattern.
The servo area includes a preamble region for clock synchronization, a SAM region for indicating the start of servo data, a track address signal region for indicating the track number, a sector address signal region for indicting the sector number, and a burst signal region for tracking the magnetic head.
In the burst signal region, a number of burst signal unit portions are magnetically recorded in a plurality of types of separate burst signal groups. Each burst signal group includes a plurality of burst signal unit portions which are recorded in rows at appropriate intervals in the length-wise direction of the track (i.e., in the relative direction of travel of the magnetic head). Now, for understanding of the present invention, a more specific description will be made to the configuration and operation of the burst signal unit portion in accordance with an example in which four types of burst signal groups are defined.
First and second burst signal groups are magnetically recorded such that the centers of the burst signal unit portions thereof in the direction of track width are displaced relative to each other by one track pitch in the direction of the track width. Likewise, third and fourth burst signal groups are also magnetically recorded such that the centers of the burst signal unit portions thereof in the direction of track width are displaced relative to each other by one track pitch in the direction of track width. However, relative to the centers of the burst signal unit portions of the first and second burst signal groups in the direction of track width, the centers of the burst signal unit portions of the third and fourth burst signal groups are also displaced by a half of the track pitch in the direction of track width. In the foregoing, a number of each burst signal groups are recorded at intervals of two track pitches in the direction of track width. The positional relation between the aforementioned four types of burst signal groups is that between neighboring four types of burst signal groups.
While traveling along (or relative to) a track on which data is to be recorded or reproduced, the magnetic head detects the outputs from the burst signal unit portions of the first and second burst signal groups corresponding to this track. The differential value between the outputs from the burst signal unit portions of the first and second burst signal groups varies according to the amount of displacement of the magnetic head relative to the track in the direction of track width. Accordingly, based on the differential value, it is possible to compensate for the position of the magnetic head in the direction of track width by knowing the amount of displacement of the magnetic head in the direction of track width relative to the track on which data is to be recorded or reproduced. On the other hand, when only the first and second burst signal groups are not enough to accurately position the magnetic head, it is also possible to use the third and fourth burst signal groups in order to position the magnetic head with improved accuracy.
The arrangement of the burst signal unit portions shown here is only an example in which four types of burst signal groups are defined. In another example, the burst signal unit portions may also be recorded such that the first and second burst signal groups are displaced relative to each other by two-thirds of the track pitch in the direction of track width while the third and fourth burst signal groups are also displaced relative to each other by two-thirds of the track pitch in the direction of track width. Additionally, the third and fourth burst signal groups are displaced relative to the first and second burst signal groups by one-third of the track pitch in the direction of track width. Alternatively, with the third and fourth burst signal groups eliminated, only the first and second burst signal groups may be magnetically recorded. On the other hand, it may also be acceptable to magnetically record six or eight types of burst signal groups.
In general, the servo signal unit portion is recorded by the servo track writing method in a rectangular shape on a magnetic recording medium. At this time, since the magnetic head moves in the radial direction(the direction of track width) along an arcuate path over the magnetic recording medium, the servo signal unit portions are magnetically recorded with the maximum angular difference of about 20 degrees depending on the radial position. However, since the magnetic head also moves along an arcuate path in the radial direction over the magnetic recording medium in the magnetic recording/reproducing apparatus, it is possible to accurately read the output from the servo signal unit portions that have been magnetically recorded with an angular difference depending on the radial position.
However, such a servo signal recording step raised a problem of low productivity because in this step, the servo signal unit portions and their surrounding portions are magnetized in opposite polarities one by one in each magnetic recording medium. Recently, among other things, because of increases in areal density and attendant decreases in flying height of the head, it has been required to record servo information with high density and accuracy. There is thus an increasing need for recording servo information with improved efficiency.
In this context, such a magnetic recording medium has been suggested in which the recording layer is formed only in either each servo signal unit portions or its surrounding portion in the servo area, with a servo pattern formed to have a geometrical feature. With this construction, servo information can be recorded with significantly improved efficiency because the recording layer is magnetized exactly in the servo pattern by uniformly applying a direct-current magnetic field to the magnetic recording medium.
By the way, magnetic recording media such as hard disks have been improved by employing finer magnetic particles forming the recording layer, alternative materials, and finer processing of the head, thereby attaining a significant increase in areal density. Further expectations are also placed on the improvement of areal density. However, these conventional approaches to the improvement of areal density have already reached their limits due to the limited accuracy in processing the magnetic head or recording to a track adjacent to a target track or crosstalk on reproducing operation caused by a spread of the recording magnetic field of the magnetic head. Accordingly, candidates for a magnetic recording medium have been suggested which are capable of attaining further improved areal density. These candidates include a discrete track medium or patterned medium in which the recording layer is formed in a concavo-convex pattern in the data area, with the recording elements formed as a convex portion of the concavo-convex pattern. To manufacture such a discrete track medium or patterned medium, there is a step of processing the recording layer to form a recording elements as a convex portions in the data area. In this step, the servo signal unit portions can also be formed at the same time either as a convex or concave portions in the servo area. This is advantageous especially in terms of productivity (e.g., see Japanese Patent Laid-Open Publication No. Hei 6-195907).
To process the recording layer into a servo pattern or track pattern, a mask layer or a resist layer may be formed on a continuous recording layer, and the servo pattern or track pattern is then formed on the resist layer by lithography or nano-imprint. Subsequently, the mask layer and the recording layer are sequentially processed by dry etching.
However, it was not easy to form a desired servo pattern with accuracy in the resist layer. For example, a concavo-convex shape corresponding to such a complicated servo pattern was not easy to form in the resist layer by lithography, and the concavo-convex shape was thus difficult to form with sufficient accuracy. On the other hand, even in the case of using a stamper, since the stamper is also manufactured by lithography which has a transfer surface of a concavo-convex shape corresponding to that of the recording layer, there was a problem that the concavo-convex shape could not be formed on the stamper with sufficient accuracy. Furthermore, even by dry etching, it was difficult to process the mask layer or the recording layer with accuracy exactly according to the shape of the patterned resist layer. That is, it is not easy to accurately form a rectangular servo signal unit portions as a convex or concave portions exactly in the desired shape. In practice, the servo signal unit portions may be formed in an unintended contour or at an unintended position or angle.
In particular, it is difficult to precisely form the rectangular servo signal unit portions exactly according to a complicated servo pattern, having an angular difference depending on the radial position, corresponding to the arcuate path of the magnetic head of the magnetic recording/reproducing apparatus. Thus, the servo signal unit portions tend to be formed at an unintended angle. As such, the servo signal unit portions disposed at an unintended angle may cause an unallowable angular difference between the servo signal unit portions and the magnetic head, leading to degradation in read accuracy of the servo information and thus resulting in an error.
More specifically, magnetic recording media, such as the discrete track medium and the patterned medium, which have high areal densities, are subject to tracking error due to degradation in accuracy in reading burst signals.